The field of the present disclosure relates generally to rotary machines, and more particularly to airfoils used with rotary machines.
At least some known rotary machines such as, gas turbine engines used for aircraft propulsion, include a plurality of rotating blades that channel air downstream. Each blade has a cross-sectional shape that defines an airfoil section. Conventional single rotation turboprop engines provide high efficiency at low cruise speeds (flight Mach number up to about 0.7), although some single rotation turboprop engines have been considered for higher cruise speeds. Higher cruise speeds (Mach 0.7 to 0.9) are typically achieved using a ducted turbofan engine to produce the relatively high thrust required.
Unducted, counter-rotating propeller engines, frequently referred to as the unducted fan (UDF®), or open-rotor, have been developed to deliver the high thrust required for high cruise speeds with higher efficiency than ducted turbofans. Counter-rotating propellers for high cruise speed efficiency have strong acoustic interactions (i.e., noise generation) at low flight speed, such as takeoff, typically at flight Mach number of 0.3 or less. Counter-rotating propellers designed for quiet operation at low flight speed tend to be inefficient at high cruise speeds. Thus, a need exists for both single rotation and counter-rotating propellers that have both good efficiency at high flight speed and low noise at low flight speed.
To operate at a wide range of operating conditions, propeller blades are typically attached to rotating hubs such that each blade setting angle, or pitch, can be adjusted during flight. Although this adjustment of blade pitch angle affects performance, because the blades are essentially rigid, the airfoil sections that comprise a blade are shaped in a specific way to improve both efficiency at high speed flight and reduce noise at low speed flight. Thus, a need exists for propellers that have both high efficiency and low noise at high speed.